This invention relates to withdrawing a slurry of a solid in a liquid from a flowing stream of the slurry.
Addition polymerizations are frequently carried out in a liquid which is a solvent for the resulting polymer. When high density (linear) ethylene polymers first became commercially available in the 1950""s this was the method used. It was soon discovered that a more efficient way to produce such polymers was to carry out the polymerization under slurry conditions. More specifically, the polymerization technique of choice became continuous slurry polymerization in a pipe loop reactor with the product being taken off by means of settling legs which operated on a batch principle to recover product. This technique has enjoyed international success with billions of pounds of ethylene polymers being so produced annually.
One problem presented by this technique relates to the matter of xe2x80x9cfines.xe2x80x9d The produced polymer is in the form of particles of varying sizes suspended in the diluent. The smaller particles are referred to as fines. The fines may be the result of many factors. Some polymer particles form during a single pass through the reactor loop and exit the first time they come to a take off point. Such particles may be smaller because of their shorter time in the reaction zone. Other particles make varying numbers of loops before being withdrawn. For these particles, some may be physically broken up by contact with the pump impeller. Others may be in diluent which is heated by friction at the impeller surface to the point that bubbles are formed, which bubbles later collapse and fracture polymer particles. Other fines may be the result of smaller catalyst particles.
The mechanism by which the polymer is formed involves a catalyst site adding together literally thousands of monomer units in a fraction of a second and thereafter terminating the polymer chain and starting another. Hence, the small particles do not represent lower molecular weight materialxe2x80x94the molecular weight is controlled by well known process variables. Thus the polymer molecules making up large particles and small particles are of essentially the same molecular weight. Rather, the fines are undesirable because they interfere with downstream polymer finishing.
All of this is to be distinguished from fines that may result downstream in the polymer finishing operation where diluent containing a small amount of low molecular weight (and hence soluble) polymer is flashed with the result that the low molecular weight material precipitates out as low molecular weight fines. Because they are in admixture with vapor these low molecular weight fines can be separated from the vapor with a cyclone separator.
Cyclone separators which separate solids from vapors are to be distinguished from hydrocyclone separators, sometimes referred to as liquid separators. These hydrocyclone separators stratify solid particles in a liquid slurry based on particle size.
The settling legs themselves also can present problems. First, they represent the imposition of a xe2x80x9cbatchxe2x80x9d technique onto a basically continuous process. Each time a settling leg reaches the stage where it xe2x80x9cdumpsxe2x80x9d or xe2x80x9cfiresxe2x80x9d accumulated polymer slurry, it causes an interference with the flow of slurry in the loop reactor upstream and the recovery system downstream. Also, conventional settling legs have sections in which polymer can collect while waiting for the next dump cycle and such collected polymer can melt over time and deposit on the inside walls of the settling leg.
In spite of these limitations, settling legs continue to be employed. This is because, as the name implies, settling occurs in the legs to thus increase the solids concentration of the slurry finally recovered as product.
Slurry can be withdrawn on a continuous basis by taking advantage of the fact that without the periodic upsets caused by settling leg firings, a higher overall reactor concentration can be achieved.
It is an object of one aspect of this invention to separate fines from a slurry reactor effluent by means of a hydrocyclone and recycle the fines to the reaction zone;
It is another object of this invention to reduce the energy required to separate diluent from product polymer solids; and
It is yet another object of this invention to avoid polymer build-up in the take off means;
In accordance with one embodiment of this invention, slurry comprising polymer and diluent is withdrawn from a flowing stream and passed to a hydrocyclone where fines and a portion of the diluent are separated from the remaining slurry as overhead and recycled to the reaction zone. In accordance with one aspect of this embodiment, the hydrocyclone bottom slurry is passed to a solid-liquid separator where additional diluent is separated from the polymer. In accordance with another aspect of this embodiment, the hydrocyclone bottoms slurry is passed through a heated flash line prior to further downstream processing.
In accordance with another embodiment of this invention, an olefin polymerization is carried out in a loop reaction zone under conditions of high solids concentration and a portion of the circulating slurry is withdrawn through a tapered settling leg.